how long do they pip?

Two eggs have the beginnings of openings ...it's day 20ish and we already hatched a duck. I think the chicks might be coming. but how long do they pip before they get out???
They are under Momma Chick....

Sunday morning brought me a cutie little chick that is light brown with dark marks and monday morn brought me another chick who is dark brown with a little lighter head...sorry my pics won't upload .....
Thank God they are here! There are still five or six more eggs under Momma. Two have started zipping. One is three quarters done and one is just starting....how long will this go on???? It is sooooo exciting!!!!

That is a tough decision. It is getting late tonight. I usually don't try to help unless no progress has been made for over 6 hours. There is a sticky in this forum that tells how to help if necessary.

well sad news...those two that I was waiting on were stinking and they didn't make it. I think they might have needed help but it was too late.....
there are still 4 more eggs under Momma so I will watch carefully....
thanks for your help!